1. Field of the Invention
The present invention relates to polytetrafluoroethylene fine particles and powder.
2. Description of the Related Art
Polytetrafluoroethylene (PTFE) is commercially available in the form of fine particles or powder as a molding material, and its molded articles are used in various fields since PTFE is excellent in heat and cold resistance, flame retardance, slidability, non-tackiness, soil-proofing, chemical resistance, weather resistance, electrical properties and the like. Further, PTFE fine particles or powder are used as a modifier by dispersing in or blending with other materials.
For blend with a structural material such as a molding resin or elastomer, PTFE powder is used, while for dispersion in a liquid or semi-solid material such as a coating material, a paint, an oil, a grease or an ink, fine particles or powder of PTFE are used.
The PTFE fine particles are in a colloidal form having an average particle size of 0.05 to 1.0 .mu.m and prepared by emulsion polymerization of tetrafluoroethylene in an aqueous medium in the presence of a surfactant. Usually, they are available in the form of an aqueous dispersion (latex). On the other hand, the powder type PTFE includes a coagulated powder (fine powder) of PTFE fine particles which is prepared by coagulating the above aqueous dispersion and drying the coagulated product and has an average particle size of 100 to 1000 .mu.m and granular particles (granular or molding powder) which is prepared by suspension polymerization of tetrafluoroethylene in the presence of a small amount of a surfactant or in the absence of a surfactant. Both powders are widely used as molding powders. Among the above powders, some powders which are heated treated at a temperature higher than a melting point of PTFE are commercially distributed. Such powders are prepared by heating the raw powder at a temperature higher than the melting point and then pulverizing it.
The fine powder or the molding powder which is used as a raw material of molding consists of PTFE having a sufficiently high molecular weight and seldomly used as a modifier of other materials by dispersion or blend except in very special cases. The major reason why these powders are not suitable for dispersion or blend is that the powders are fibrilled during dispersing or blending.
Fibrillation of PTFE appears, in particular, in a virgin powder which has not been subjected to the heat treatment at a temperature higher than the PTFE melting point after polymerization, inter alia, the colloidal fine particles and the fine powder which are prepared by emulsion polymerization.
The PTFE fine powder is usually molded by "paste extrusion" comprising mixing the powder with a lubrication aid to form a paste, filling the paste form powder in a cylinder having a thin orifice and cold extruding it. The extruded article is sintered to form a tube or a wire coating, or pressed in a film form to produce a sealing tape. Further, after pressing, the film is stretched to produce a porous film.
The paste extrusion is based on the fibrillation of the colloidal PTFE fine particles. Since the PTFE particles are fibrilled and tangled with each other during extrusion, mechanical strength is imparted to the molded article before heat treatment to some extent. However, since the PTFE is easily fibrilled with a small shear force, when the PTFE fine powder is blended with other resin or elastomer, it is fibrilled by a shear force generated by blending, whereby a viscosity of a mixture increases greatly so that no uniform mixing is possible.
When the PTFE latex is mixed with other materials preferably with paint or the like, the colloidal PTFE fine particles are difficult to redisperse if they are once coagulated, so that a uniform dispersion state of the particles is not maintained.
The fibrillation property of the virgin PTFE, in particular, the colloidal fine particles depends on the molecular weight of PTFE and the fibrillation occurs in PTFE having a molecular weight higher than a certain value, and does not in PTFE having a low molecular weight. Then, fine particles and powder of a low molecular weight PTFE which is prepared by polymerizing tetrafluoroethylene under conditions to achieve the low molecular weight have been commercially sold.
When the high molecular weight PTFE is heated, usually at a temperature higher than the melting point, the fibrillation property almost disappears. This may be due to the difference of crystal structure between before and after heat treatment, that is, between the virgin PTFE and sintered PTFE. Irrespective of the molecular weight, the sintered PTFE has substantially no fibrillation property.
Accordingly, for dispersing in or blending with the molding resin material or the paint, the fine particles or powder of the low molecular weight PTFE or the high molecular weight PTFE which has been heat treated are suitable.
In general, polymers having a higher molecular weight have more preferable mechanical properties. For the improvement of slidability which is one main object of the dispersion or blend of fine particles or powder of PTFE, the addition of the higher molecular weight PTFE seems to achieve better abrasion resistance. Then, it is desired to produce fine particles or powder of PTFE which has a high molecular weight and is hardly fibrilled.
Though sintered PTFE having a high molecular weight is not produced in the form of colloidal fine particles, the powder form sintered PTFE, for example, high molecular weight PTFE such as molding powder which is sintered and pulverized is commercially sold. However, the high molecular weight virgin PTFE which has been sintered cannot be pulverized to a particle size of less than 70 .mu.m by the usual mill or pulverizer, except that it is comminuted by a very special uneconomical method such as freeze comminuting. By the usual pulverizer, the produced particles have large anisotropy in shape, so that they are not practically usable fine particles for dispersion or blend. The commercially available sintered PTFE particles for molding has an average particle size of at least 100 .mu.m. Though the commercially available sintered PTFE type powder for dispersion or blend is in the form of minute particles having a particle size of 10 to 40 .mu.m, it is produced by subjecting scraps of sintered molded articles to an expensive treatment, for example, thermal decomposition or irradiation to decrease the molecular weight or by heating low molecular weight PTFE and pulverizing it. The sintered PTFE type powder produced by either of the above two methods has a low molecular weight.
Irrespective of the virgin PTFE or the sintered PTFE, the fine particles or powder of PTFE suitable for dispersion or blend has in general a low molecular weight and has a melt viscosity at 380.degree. C. of about 1.times.10.sup.2 to 1.times. 10.sup.6 poises, while the molding PTFE usually has a melt viscosity at 380.degree. C. of at least 1.times.10.sup.8 poises, preferably 1.times.10.sup.11 to 1.times.10.sup.12 poises.
For dispersion or blend, the fine particles or the powder of the virgin or sintered PTFE is used according to applications. In general, the virgin PTFE powder is soft and tends to be deformed during blending, while the sintered PTFE powder is hard and dense so that it often keeps the particle shape in the blend. Since the PTFE fine particles are dispersed by latex blending, they are more easily microdispersed than the powder.
To solve the above problems, various proposals have been made. For example, it is proposed to suppress the fibrillation characteristics while keeping the high molecular weight by providing fine particles, each having a core/shell structure, by coating surfaces of the colloidal PTFE fine particles with a different resin which is not fibrilled. Japanese Patent Kokai Publication No. 109846/1987 discloses fluorine-containing complex resin fine particles, each comprising a core made of PTFE and a shell made of a copolymer of tetrafluoroethylene (TFE) and chlorotrifluoroethylene (CTFE). Also, Japanese Patent Kokai Publication No. 284201 discloses similar core/shell particles comprising a core made of PTFE and a shell of an acrylic polymer. Both publications disclose the use of these core/shell particles in the form of an organosol which is easily redispersible. The invention of Japanese Patent Kokai Publication No. 247408/1989 is based on the same technical idea as Japanese Patent Kokai Publication No. 109846/1987 and intends to provide a powder suitable for blending with a resin or an elastomer by suppressing fibrillation of the core polymer through copolymerization of a modifying amount of hexafluoropropylene, perfluorobutylethylene, perfluoromethyl vinyl ether or perfluoropropyl vinyl ether.
However, though either one of the above prior art core/shell particles can be used in one specific application, a sintered type high molecular weight powder for a blend cannot be obtained in the form of spherical shape particles by coagulating the aqueous dispersion, drying the recovered polymer particles, heating them and then pulverizing them since the particle size is greatly decreased and the particle shape becomes whisker like.